Throttle sticking when hot

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Has anyone had this happen?

My 74 Norton has a single Amal MK2 installed. When the engine is nicely warmed up, after maybe 5 to 10 miles, the throttle begins to stick. It begins with a little twitch then progresses to the throttle sticking open, not responding to the twistgrip being returned. Snapping the twistgrip and popping the clutch does no good. It won't be idled while running and will only function normally when the engine is shut down and allowed to cool. As soon as it heats up again, sticking starts all over again. If turned off while hot then restarted, it operates fine up to about 2500 RPM then sticks open once that threshold is passed. Cruising down the road at a steady 5K RPM is exciting to say the least.

I've checked the cable linkage and the twist grip and they seem to be operating with very little friction. I suspect that the carburetor throat is warping slightly when the engine/carb heats up and the warping is causing the slide to stick open.

I've tried a new slide spring and a new chrome slide but it's still not working.

Any ideas?

Brass slide chrome plated always does the trick with Mark2's. You have a fiberglass tank? I would be looking for fuel contamination. Melting plastic button for the slide key OK? The neddle clean also it's jet is it good and round. Hope you replaced both when you did the brass slide thing. My first set of neddles and jets went only 500 miles before the dampening was added. (see Mark 2 article) Pull the manifold look at the intake valves for on odd coating if that's alright check the manifold for flat. Cables must be very light gage or galling will start in. norbsa
Amal MK2 Slide Sticking

Thanks norbsa48503 for your reply. Steel Interstate tank newly tinned. Slide button functioning properly (took it out and examined it). Needle and jet look new. The carb is nearly new with only 1500 miles on it. Which "MK2 article" are you referring to?

Seems that I've found a quality problem. It's the MK2 made in Spain. Examined the sides of the tube the slide moves in. Noticed several deep concentric machining/boring grooves in two different places. Moving the slide up and back over these grooves produces a very slight sticking. Since the position of the grooves is consistent with where the throttle is sticking, I'm concluding that the grooves are what's causing the throttle to stick.

That's the second quality problem I've had to solve on this same carb...the first was a float with a needle tab that wasn't sturdy enough to close off the fuel supply. It was too flexible and was causing severe flooding. Adjusting the seat up and down had no effect. Relaced a viton tipped needle, no effect. Finally replaced the float with one from and earlier model and it worked perfectly. Seems the type of plastic used in the newer floats is not as stiff as prior designs.

I'm considering having it sleeved...does anyone know if there's a stainless steel sleeve available? Who has it?


Yes you are in the very early stages of mark two set up. Look on this site for " Setting up mark two's" There are a number of things you need to do to get them right this may help. Bruce in Canada at will set the bores straight for you see his web site. He has a really fine fastback package rail as well. Hand made light wt. Cables are a must the lighter the better. norbsa
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